1. Field of the Invention
The present invention relates to a diaphragm pump and a blood pressure monitor.
2. Description of the Background Art
In recent years, importance has increasingly been placed on self-management of a blood pressure and blood pressure monitors for home use have widely been available. In measuring a blood pressure, an arm band containing a bladder is wound around a part of a living body and air is sent into the bladder for pressurization. A blood pressure is measured based on artery information obtained by pressing the living body. For pressurization of the bladder, an air pump is used. The air pump includes a rubber-like diaphragm portion forming a pump chamber within a case, a piston provided in the diaphragm portion, and a drive element attached to the piston. As the drive element swings to cause the piston to carry out reciprocating motion, a volume of the pump chamber is varied. As a result of change in volume of the pump chamber, a pumping action for transferring the air is performed. The air pump is provided with a check valve, in order to avoid back flow of exhausted air and intake air.
Air is continuously exhausted from a tip end of the pump, in accordance with change in volume of the pump chamber. As the piston moves downward, air flows into the pump chamber, and as the piston moves upward, air is exhausted from the pump chamber. As the piston repeats up-down movement, pressure fluctuation of a gas discharged from the pump (hereinafter referred to as pressure ripple) is generated. As the pressure ripple of the pump is generated, its amplitude makes heaving sound (pressurization sound) larger and pump noise increases.
The pump included in the blood pressure monitor is connected to an air valve, a pressure sensor, and an arm band through an air tube. A pressure pulse wave of a human body detected at the arm band is measured by the pressure sensor. Here, in order to conduct accurate blood pressure measurement, pressure ripple generated from the pump should be eliminated to measure a pure pressure pulse wave of the human body. Therefore, various techniques for eliminating pressure ripple have conventionally been proposed (see, for example, Japanese Patent Laying-Open No. 2003-162283 (Patent Document 1), Japanese Patent Laying-Open No. 7-210167 (Patent Document 2), and Japanese Patent Laying-Open No. 11-276447 (Patent Document 3)).
Japanese Patent Laying-Open No. 2003-162283 (Patent Document 1) proposes an acoustic filter including a tank and a thin pipe, in which the tank and the thin pipe are formed by layering filter cases. Japanese Patent Laying-Open No. 7-210167 (Patent Document 2) proposes an acoustic filter including a tank and a thin pipe, in which the tank and the thin pipe are formed by layering a filter case and packing.
Japanese Patent Laying-Open No. 11-276447 (Patent Document 3) proposes a blood pressure monitor including a low-cut filter for outputting a ripple component of an air pump, a ripple suppression comparator receiving an output from the low-cut filter as an input, and an adder for adding the output from the ripple suppression comparator to an output from a volume compensation comparator. In addition, a method for eliminating a pump ripple signal in a blood pressure monitor, by causing a signal indicating a pressure within a cuff obtained by the pressure sensor to pass through the low-cut filter so as to obtain only the ripple component of the pump without including a blood pressure signal component, comparing a signal from the low-cut filter with a reference value of zero by using the ripple suppression comparator, adding an error signal output from the ripple suppression comparator to an error signal from the volume compensation comparator, inputting the resultant signal to a valve controller, and controlling a leakage valve, has been proposed.
Conventional methods proposed in Japanese Patent Laying-Open No. 2003-162283 (Patent Document 1), Japanese Patent Laying-Open No. 7-210167 (Patent Document 2), and Japanese Patent Laying-Open No. 11-276447 (Patent Document 3), however, are not methods for reducing the pressure ripple itself generated from the pump. Therefore, such a problem that pressurization noise due to the pressure ripple of the pump is large still remains. In addition, as the acoustic filter is provided at a pressure sensor inlet portion, such problems as complicated air pipe arrangement, increase in the number of parts, and cost increase arise. Moreover, the method described in Japanese Patent Laying-Open No. 11-276447 (Patent Document 3) requires the ripple suppression comparator, the low-cut filter, the valve controller, and the like, and the number of air system parts increases, which results in complicatedness.